Abstract
Two class 1 integrons, In-t1 and In-t2, were previously identified in IncFI plasmids of
Salmonella enterica
serotype Typhimurium. Molecular analysis revealed a close physical link between the two
integrons. In-t1 is preceded by the transposase genes of Tn
21
, whereas In-t2 is located downstream the 3′-conserved segment (3′-CS) of In-t1, in a head-to-tail configuration. In-t1 shows
a peculiar sequence downstream the 3′-CS, containing an extended version of the open reading frame known as ORF341 (referred to as ORF341E) and a novel trimethoprim resistance gene, designated
dfrA18.
Retrospective analysis provided evidence for In-t1 insertion within Tn
1935
, a Tn
21
-related transposon identified in IncFI plasmids circulating among epidemic clones of multidrug-resistant
S. enterica
during the 1970s. Structural comparison between Tn
21
derivatives from recent and ancestor IncFI plasmids showed that In-t2 has been conserved by these replicons. In-t1 belongs
to a novel family of class 1 integrons containing the ORF341E sequence, and appears to have been acquired by IncFI plasmids after the assembly of Tn
1935
. In-t1 insertion occurred within the 5′-conserved
segment (5′-CS) proximal region of the resident In-t2.